This invention relates to an operational amplifier provided with a phase compensation circuit.
To date, various phase compensation circuits have been proposed to assure a stable operation in a high frequency region.
An operational amplifier apparatus is generally formed of two cascade-connected amplifiers. However, such type of operational amplifier apparatus tends to be unstable in the broad band. A known process for eliminating the drawback is to connect a capacitor between the output terminal of a second one of the two cascade-connected amplifiers and a node or joint of the first and second amplifiers (FIG. 1). FIG. 2 illustrates the gain characteristic of the conventional operational amplifier apparatus, showing the presence of three poles P.sub.1, P.sub.2 and P.sub.3. Now let it be assumed that the three pole frequencies are respectively represented by f.sub.P1, f.sub.P2 and f.sub.P3. Then f.sub.P2 and f.sub.P3 are respectively expressed by the following equations: ##EQU1## The unity gain frequency f.sub.0dB is expressed by the following equation: ##EQU2##
The unity gain frequency f.sub.0dB should have a value expressed as f.sub.0dB &lt;&lt;f.sub.P2 and f.sub.0dB &lt;&lt;f.sub.P3 in order to let the operational amplifier have a sufficient phase margin and carry out a stable operation in the broad band.
However, the transconductance of each operational amplifier constituted by MOSFETs ranges from several .mu.mho to hundreds of .mu.mho, and consequently presents difficulties in meeting the above-mentioned requirement. Assuming, for example, capacitance C.sub.3 =50 pF, conductance g.sub.3 =200 .mu.mho, then there results frequency f.sub.P3 .perspectiveto.637 kHz. Therefore, it is difficult to realize a broad band amplifier whose unity again frequency is expressed as f.sub.0dB .perspectiveto.1 MHz. Further, it is necessary to let the MOSFET of the output buffer stage A.sub.3 have a great channel width in order to enlarge the conductance g.sub.3 for an increase in the frequency f.sub.P3. An attempt to satisfy such requirement is accompanied with the drawbacks of presenting difficulties in integrating the subject operational amplifier apparatus and increasing its lower power consumption.
The Japanese patent laid-open application No. 52-49,755 sets forth another process of resolving the above-mentioned drawbacks. As shown in FIG. 3, this prior art operational amplifier comprises a first amplifier A.sub.1, second amplifier A.sub.2 and output stage A.sub.3. An output signal from this output stage A.sub.3 is negatively fed back to the junction of the first and second amplifiers. The operational amplifier of the Japanese patent laid-open application No. 52-49,755 arranged as described above has the advantage of eliminating various drawbacks accompanying the conventional amplifier of FIG. 1. Where, however, a MOSFET is used as an amplifying element, the gain characteristic of the operational amplifier of the Japanese patent laid-open application No. 52-49,755 is handicapped by the fact that a quadratic pole tends to arise. FIG. 4 illustrates the gain characteristic of a general operational amplifier having a quadratic pole. The gain characteristic tends to indicate a peak in the proximity of a unity gain frequency f.sub.0dB. This peak results from a quadratic pole frequency f.sub.Pd. At this quadratic pole frequency f.sub.Pd, the phase is suddenly rotated through 180.degree., leaving no phase margin. Only where, therefore, the condition of f.sub.0dB &lt;&lt;f.sub.Pd is established, it has been impossible in the past to realize a broad band amplifier. The above-mentioned quadratic pole frequency f.sub.Pd is expressed as ##EQU3##
To eliminate the peaking phenomenon, it is necessary to let a certain constant .delta. on an equivalent circuit have a larger value than 1. In this case the constant .delta. is expressed as: ##EQU4## r.sub.2 denotes an output resistance of the second operational amplifier A.sub.2. If it is tried to let the capacitance C.sub.3, the conductances g.sub.2 and g.sub.3 have such a value as meets the requisite condition of the constant .delta., then the quadratic pole frequency f.sub.Pd decreases, failing to improve the phase margin. The capacitor C.sub.2 represents a sort of parasitic capacitance, and consequently has little latitude to be modified for the design and manufacture of an operational amplifier. Further, an attempt to reduce an output resistance r.sub.2 results in a decline in the DC gain of the second operational amplifier. The DC gain is expressed as -g.sub.2 r.sub.2. Therefore, it is necessary to let g.sub.2 have a large value in order to decrease the output resistance r.sub.2 without reducing the DC gain. This means that the MOSFET of the second operational amplifier A.sub.2 must have an enlarged channel width, undesirably reducing the latitude with which the subject operational amplifier can be integrated.